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AD8350AR20-REEL7 Datasheet(PDF) 9 Page - Analog Devices |
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AD8350AR20-REEL7 Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page REV. A AD8350 –9– For the output matching network, if the output source resis- tance of the AD8350 is greater than the terminating load resistance, a step-down network should be employed as shown on the output of Figure 3. For a step-down matching network, the series and parallel reactances are calculated as: X RR X XR R RR S S LOAD P PS LOAD S LOAD = × =× where – (2) For a 10 MHz application with the 200 Ω output source resistance of the AD8350, RS = 200 Ω, and a 50 Ω load termination, RLOAD = 50 Ω, then XP = 115.5 Ω and XS = 86.6 Ω, which results in the following component values: CP = (2 π × 10 × 106 × 115.5)–1 = 138 pF and LS = 86.6 × (2 π × 10 × 106)–1 = 1.38 µH The same results can be obtained using the plots in Figure 5 and Figure 6. Figure 5 shows the normalized shunt reactance versus the normalized source resistance for a step-up matching network, RS < RLOAD. By inspection, the appropriate reactance can be found for a given value of RS/RLOAD. The series reactance is then calculated using XS = RS RLOAD/XP. The same technique can be used to design the step-down matching network using Figure 6. 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 NORMALIZED SOURCE RESISTANCE – RSOURCE/R LOAD RSOURCE XS RLOAD XP Figure 5. Normalized Step-Up Matching Components NORMALIZED SOURCE RESISTANCE – RSOURCE/R LOAD 3.2 3 2.8 2.6 2.4 2.2 2 RSOURCE XS RLOAD XP Figure 6. Normalized Step-Down Matching Components The same results could be found using a Smith Chart as shown in Figure 7. In this example, a shunt capacitor and a series inductor are used to match the 200 Ω source to a 50 Ω load. For a fre- quency of 10 MHz, the same capacitor and inductor values previously found using the resonant approach will transform the 200 Ω source to match the 50 Ω load. At frequencies exceeding 100 MHz, the S parameters from Tables II and III should be used to account for the complex impedance relationships. SOURCE LOAD SHUNT C SERIES L Figure 7. Smith Chart Representation of Step-Down Network After determining the matching network for the single-ended equivalent circuit, the matching elements need to be applied in a differential manner. The series reactance needs to be split such that the final network is balanced. In the previous examples, this simply translates to splitting the series inductor into two equal halves as shown in Figure 3. Gain Adjustment The effective gain of the AD8350 can be reduced using a num- ber of techniques. Obviously a matched attenuator network will reduce the effective gain, but this requires the addition of a separate component which can be prohibitive in size and cost. The attenuator will also increase the effective noise figure resulting in an SNR degradation. A simple voltage divider can be imple- mented using the combination of the driving impedance of the previous stage and a shunt resistor across the inputs of the AD8350 as shown in Figure 8. This provides a compact solution but suffers from an increased noise spectral density at the input of the AD8350 due to the thermal noise contribution of the shunt resistor. The input impedance can be dynamically altered through the use of feedback resistors as shown in Figure 9. This will result in a similar attenuation of the input signal by virtue of the voltage divider established from the driving source imped- ance and the reduced input impedance of the AD8350. Yet this technique does not significantly degrade the SNR with the unnecessary increase in thermal noise that arises from a truly resistive attenuator network. |
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